1. Technical Field of the Invention
The present invention relates to a method and apparatus for bidirectional current sensing for high power full and half-bridge transistor circuits, and more particularly, to such a method and apparatus which no insertion of elements into the power circuit, and which can be fully integrated into the bridge driver IC.
The invention will be described in the context of a half-bridge MOSFET circuit, but should be understood that the concepts disclosed are also directly applicable to circuits employing other types or power transistors, ane to circuits in full-bridge topology.
2. Related Art
High voltage half-bridge and full-bridge power circuits are used in various applications such as motor drives, electronic ballasts for fluorescent lamps and power supplies. A simple half-bridge circuit is illustrated at 10 in FIG. 1. The circuit employs high-side and low-side totem pole connected MOSFETS MHS and MLS that feed a load 12 connected to a common point 14 between the MOSFETS, and a second common point 16 between two capacitors C1 and C2, through a first circuit path including MOSFET MHS and capacitor C1, and a second circuit path including MOSFET MLS and capacitor C2.
A gate drive circuit 18, typically an IC, supplies gate drive signals to turn MOSFETS MHS and MLS on or off in response logic input signals HIN and LIN.
In power applications involving a half bridge or full bridge topology, current sensing is often needed for feedback or to prevent run away situations. Conventional methods of current sensing require insertion of external elements such as resistors into the power circuit, e.g. between MOSFET MHS and MLS (not illustrated), which are intrusive, i.e. cause power dissipation, and require extra pins on the driver IC.
Moreover, in some high power bridge topology applications, e.g., Class-D amplifiers, the power transistors are connected between plus and minus voltage rails, rather than a high voltage rail and ground as illustrated in FIG. 1. In such cases, both positive and negative currents may be encountered. The measuring circuitry in the driver IC must be able to accommodate the negative swings if is to be usable in all power bridge applications.
Thus, a need exists for a circuit implementation and method which avoids the need for insertion of a sensing element in the power circuit, can be integrated with the driver IC, and which is capable of measuring bidirectional currents. The present invention is intended to meet this need.